1. Field of the Invention
The present invention relates to a method for treating a waste gas containing a fluorine component and/or a chlorine component. More particularly, it relates to a method for effectively trapping a fluorine component or a chlorine component in a waste gas discharged from a process of dry etching a material such as aluminum or silicon with a gas containing fluorine or chlorine.
2. Description of the Prior Art
In recent years, with the development of various electronic equipments, there have been widely used silicon as an electronic substrate, and a metal such as aluminum as an electrode material. Such silicon or aluminum is subjected to etching treatment as the case requires. As an etching agent for such treatment, there may be employed a chlorine component or fluorine component such as BCl.sub.3, SiCl.sub.4, CF.sub.4, CHF.sub.3 or CCl.sub.4. After the etching treatment, a waste gas containing such a chlorine or fluorine component is discharged. Although the amount of such a waste gas is not very much, the concentration of the chlorine or fluorine component in the gas is relatively high, and it is not allowable to discharge such a gas to the atmosphere from the viewpoint of the prevention of air pollution.
Heretofore, gas adsorption by means of e.g. active carbon has been regarded as being effective as a means for treating such a waste gas, since it does not produce a waste liquid.
However, so-called dry etching is commonly employed as the etching means, whereby the treating gas is forcibly discharged by a vacuum pump from the etching plant, and when the etching treatment is not in operation, nitrogen gas is introduced into the system at a rate of a few liters per minute for the protection of the vacuum pump. Therefore, if active carbon is employed for the waste gas treatment, there will be a drawback that a small amount of the chlorine component or fluorine component trapped will be desorbed from the active carbon and discharged during intermissions of the etching treatment.
Further, U.S. Pat. No. 2,919,174 discloses a method for removing a halogen acid-containing gas such as HF gas by contacting it with a fluidized bed of MgO powder. However, with respect to MgO, no physical properties are given, and therefore, MgO is believed to be of common type which has a specific surface area of at most 80 m.sup.2 /g. The removal rate of the halogen-containing gas by such MgO is relatively low.
On the other hand, when a halogen-containing gas is removed by means of Mg(OH).sub.2, MgCO.sub.3, calcium silicate or quick lime, the removal rate is extremely high at the initial stage, but soon deteriorates to a considerable extent.
In the case where the halogen-containing gas is to be removed by means of Na.sub.2 CO.sub.3 or NaHCO.sub.3, there is a drawback that Na.sub.2 CO.sub.3 or NaHCO.sub.3 tends to be finely pulverized, and the pressure loss of the gas increases substantially.